The human cornea is the clear window of the eye. It provides as much as seventy-five percent of the refractive power of the eye; thus, it is of great interest to accurately assess the eye's topography (surface shape), and have the capability to map the topography particularly over the cornea, limbus and the neighboring scleral region. This shape information can be used to diagnose corneal disease (such as Keratoconus), plan vision correction therapies involving the use and fitting of contact lenses and in intra-operative procedures, and in other ophthalmic applications as those skilled in the art will appreciate.
Commonly used and conventional corneal topography systems include, for example, Placido-based systems that measure a concentric light ring pattern that is specularly reflected from the cornea, and scanning-slit systems. Placido-based systems often suffer from the inability to measure highly aberrated corneas, which can cause multiple reflections of the mires. Scanning slit-based systems operate by capturing individual images of a bright slit of light that is diffusely reflected/scattered from various surfaces of the eye. This diffusely reflected image does not have image edges as sharp as those of the specularly reflected mire images. As a result, the inherent measurement accuracy is reduced. In addition, since the slits must be scanned over some finite time period in which the eye can be moving, there may be problems in providing an exact registration of the individual images relative to the eye being measured.
Another topography measurement system that was commercially available in the 1990's was known as the PAR Corneal Topography System (PAR CTS). The PAR CTS is referred to as a rasterstereography-based system. Rasterstereography-based topography is a method of obtaining contour or topographic information where one of the cameras in a stereogrammetric pair is replaced with a light source, which projects a cyan grid of parallel lines onto a surface to be measured. If the topography of a cornea is to be obtained, fluorescein solution is applied onto the tear film of the cornea so that the grid pattern becomes fluorescent and can be imaged. The PAR CTS was advantageous in that its operation was not dependent on specular reflection of the grid, and therefore not dependent on high quality optical surfaces or strict alignment criteria. The PAR CTS also was an “elevation” (height) system, distinguishing it from Placido-based systems, which measure surface slope rather than elevational height directly. The interested reader is directed to U.S. Pat. Nos. 4,995,716 and 5,159,361, the disclosures of which are incorporated herein by reference in their entireties.
Although the PAR CTS was discontinued by the manufacturer in 1998, many systems were sold worldwide prior thereto, and many are still in use. Some clinicians still prefer to use the PAR CTS over other topographic systems for specific patients, despite the fact that the computer (pre-Pentium) and operating system (Windows 3.11) are, by today's standards, excruciatingly slow and cumbersome, and are no longer supported by the manufacturer.
Other recognized shortcomings of the PAR CTS were related to image processing speed and capability, and specialized image acquisition hardware having limited availability. The PAR CTS also had a long working distance and long optical layout that, made it difficult to use or integrate with surgical microscopes and other diagnostic and/or therapeutic devices, thus further limiting its utility.
Embodiments of the instant invention, described herein as an advanced Rasterstereography Corneo-scleral Topography (RCT) system, overcome the abovementioned limitations, shortcomings, and disadvantages of the prior art devices and, in particular, the original PAR CTS system. In addition, the RCT system will provide measurement and analysis capabilities that both include certain desirable features of past and currently available Placido-based, scanning slit-based, three-dimensional polar grid-based, Scheimpflug-based, or other topography measurement modalities, and others, which are not technically provided by those systems. The RCT system according to the embodiments described herein will present a modern, efficient, and efficacious device that incorporates improved hardware and software over the obsolete PAR CTS system in particular, as well as other currently available corneal topography systems.